The properties of titanium have long been recognized as a light, strong, and corrosion resistant metal, which has lead to many different approaches over the past few decades to extract titanium from its ore. These methods were summarized by Henrie [1]. Despite the many methods investigated to produce titanium, the only methods currently utilized commercially are the Kroll and Hunter processes [2, 3]. These processes utilize titanium tetrachloride (TiCl4) which is produced from the carbo-chlorination of a refined titanium dioxide (TiO2) according to the reaction:TiO2(s)+2Cl2(g)+2C(s)→TiCl4(g)+2CO(g).In the Kroll process [2] TiCl4 is reduced with molten magnesium at ≈800° C. in an atmosphere of argon. This produces metallic titanium as a spongy mass according to the reaction:2Mg(l)+TiCl4(g)→Ti(s)+2MgCl2(l)from which the excess Mg and MgCl2 is removed by volatilization, under vacuum at ≈1000° C. The MgCl2 is then separated and recycled electrolytically to produce Mg as the reductant to further reduce the TiCl4. In the Hunter process [3,4] sodium is used as a reductant according to the reaction:4Na(l)+TiCl4(g)→Ti(s)+4NaCl(l)The titanium produced by either the Kroll or Hunter processes must not only be separated from the reductant halide by vacuum distillation and/or leaching in acidified solution to free the titanium sponge for further processing to useful titanium forms, but also require the recycling of the reductant by electrolysis. Because of these multiple steps the resultant titanium is quite expensive which limits its use to cost insensitive applications.
The high cost of the Kroll process results in a high cost of titanium products limiting their widespread utilization in spite of their exceptionally desirable properties. Since titanium's discovery, investigations have been conducted to produce titanium by more economical processing other than the metalothermic reduction such as magnesium or sodium reduction of TiCl4, but without sufficient success to replace the high cost Kroll process. The intensive interest to develop low cost processing to produce titanium has recently spun several published processes. Since titanium primarily appears as the oxide (TiO2), it can be conceived that an oxide feed to produce titanium could be more economical than making the chloride (TiCl4) by carbo-chlorination of the oxide as the feed (TiCl4) which is used in the Kroll process.
The US Bureau of Mines performed extensive additional investigations [1,5-8] to improve the Kroll and Hunter processes. Many other processes have been investigated that include plasma techniques [9-13], molten chloride salt electrolytic processes [14], molten fluoride methods [15], the Goldschmidt approach [16], and alkali metal-calcium techniques [17]. Other processes investigated without measurable success have included aluminum, magnesium, carbothermic and carbo-nitrothermic reduction of TiO2 and plasma reduction of TiCl4[18]. Direct reduction of TiO2 or TiCl4 using mechanochemical processing of ball milling with appropriate reductants of Mg or calcium hydride (CaH2) also have been investigated [19] without measurable success. Kroll, who is considered as the father of the titanium industry [20] predicted that titanium will be made competitively by fusion electrolysis but to date, this has not been realized.
An electrolytic process has been reported [21] that utilizes TiO2 as a cathode and carbon or graphite as the anode in a calcium chloride electrolyte operated at 900° C. By this process, calcium is deposited on the TiO2 cathode, which reduces the TiO2 to titanium and calcium oxide. However, this process is limited by diffusion of calcium into the TiO2 cathode and the build-up of calcium oxide in the cell, which limits operating time to remove the calcium oxide or replacement of the electrolyte. Also the TiO2 cathode is not fully reduced which leaves contamination of TiO2 or reduced oxides such as TiO, mixed oxides such as calcium titanante as well as titanium carbide being formed on the surface of the cathode thus also contaminating the titanium.
In the Fray-Farthing-Chen (FFC) Cambridge process, or simply, the Fray process, titanium dioxide (TiO2) is utilized as a cathode and electrolyzed with a graphite anode in molten calcium chloride (CaCl2) which allegedly removes the oxygen from the TiO2 in pellet form leaving titanium and with the graphite anode produces CO2 at the anode. A fundamental teaching is that the oxygen ionized from the TiO2 in the cathode must be dissolved in the electrolyte which is CaCl2 for transport to the anode. In addition, it is stated that calcium titanites (CaxTiyOz) are formed as well as toxic chlorine is also given off initially at the anode. In technical public symposium, presenters of the FFC process have noted that the formation of calcium titanite is a problem to producing titanium metal and that the Columbic efficiency is very low at under 20% thus making the process expensive. Independent analysis, US Dept. of Energy Contract 4000013062 report, implies the cost of the FFC process is more expensive than the Kroll process and the product does not meet the purity of the standard Kroll material.
International patent publications WO 02/066711 A1, WO 02/083993 A1, WO 03/002785 A1 and U.S. Pat. No. 6,663,763 B2 also utilize TiO2 as a cathode feed to electrolytically extract oxygen to produce titanium metal remaining at the cathode with oxygen discharged at the anode. Each of these publications state the Fray/FFC process produces titanium with residual oxygen, carbon and calcium titanite which is unsuitable for commercial use. International patent publication WO 02/066711 A1 to Strezov et al., assigned to BHP Steel, Ltd., reports that the Fray et al. process consist of ionizing oxygen at the titania (TiO2) cathode under applied potential which oxygen removed or ionized from the TiO2 cathode is dissolved in the CaCl2 electrolyte and is transported to a graphite anode to be discharged as CO2. The first aspect of the teachings of WO 02/066711 A1 is that the electrical contact to the TiO2 cathode influences the reduction process and that a high resistive electrical conductor to the cathode is made part of the cathode. It is further reported the oxygen removed from the TiO2 cathode in a pellet form passes onto solution and/or chemically reacts with the electrolyte cation. The teaching is that deposition of the cation at the cathode is prevented through controlled potential at under 3.0V in the CaCl2 electrolyte. It is stated Al2O3 in the cathode with TiO2 can also be reduced but non-uniformly with the only reduction taking place where the Al2O3 touches the cathode conductor. The publication WO 02/066711 A1 teaches the TiO2 must be made into a pellet and presintered before use as a cathode and states the Fray et al. application mechanism is incorrect, produces 18 wt % carbon in the final titanium pellet as well as calcium titanites and silicates if silica is in the titania (TiO2) pellets. This publication claims to avoid or prevent anode material (graphite/carbon) from transport into the cathode, but provides no teaching of how this is accomplished.
International publication WO 02/083993 A1 to Stresov et al. assigned to BlueScope Steel, Ltd., formerly BHP Steel, Ltd., teaches that the electrolyte to cathodically reduce pelletized TiO2 must be calcium chloride containing CaO. This publication states that the CaCl2 electrolyte is operated to produce Ca++ cations which provide the driving force that facilitate extraction of O−− anions produced by the electrolytic reduction of titania (TiO2) at the cathode. It is reported that Ca metal exist in the electrolyte and that it is responsible for the chemical reduction of titania (TiO2). It is also reported that significant amounts of carbon are transferred from the anode to the cathode thus contaminating the titanium and was responsible for low energy efficiency of the cell. This publication teaches replacing the carbon anode with a molten metal anode of silver or copper to eliminate carbon contamination of the reduced TiO2. The teaching is that the cell potential be at least 1.5V but less than 3.0V with a cell potential above the decomposition potential of CaO. Again the titania (TiO2) cathode is in the form of a solid such as a plate.
International publication WO 03/002785 A1 to Strezov, et al., also assigned to BHP Steel, Ltd., teaches the oxygen contained in the solid form of titantia (TiO2) is ionized under electrolysis which dissolves in the CaCl2 electrolyte. It is taught that the operating cell potential is above a potential at which cations are produced which chemically reduce the cathode metal oxide/TiO2. It is further stated that chlorine (Cl2) gas is removed at the anode at potentials well below the theoretical deposition, that CaxTiyOz is present at the TiO2 cathode and that CaO is formed in the molten electrolyte bath which is CaCl2 containing oxygen ions. It is also stated the potential of the cell must vary with the concentration of oxygen in the titanium requiring higher potentials at lower concentrations of oxygen to remove the lower concentrations of oxygen. It is unlikely to remove the oxygen from TiO2 to low concentrations (i.e., 500 ppm) in a single stage operation. It is again taught that cations must be produced to chemically reduce the cathodic TiO2 requiring refreshing the electrolyte and/or changing/increasing the cell potential. The method teaches carrying out the reduction of TiO2 in a series of electrolytic cells of successively transferring the partially reduced titanium oxide to each of the cells in the series. The cell potential is above the potential at which Ca metal can be deposited via the decomposition of CaO wherein the Ca metal is dissolved in the electrolyte which migrates to the vicinity of the cathode TiO2.
In U.S. Pat. No. 6,663,763 B2 which is substantially the same as international publication WO 02/066711 A1, it is taught that CaO must be electrolyzed to produce calcium metal and Ca++ ions which reduce the titania (TiO2) in the cathode with oxygen (O=) migrating to the anode. This is very unlikely the mechanism. If Ca in metallic (Cao) or ionic (Ca++) form reduces the TiO2 the product of reduction will be CaO i.e., TiO2+2Ca=Ti+2CaO. The produced calcium from electrolysis must diffuse into the titania (TiO2) pellet to achieve chemical reduction as claimed and the formed CaO will then have to diffuse out of the Ti/TiO2 which has been preformed and sintered into a pellet. If calcium metal (Cao) or ions (Ca++) are produced by electrolysis, the oxygen ions (O=) produced from that electrolysis can diffuse to the anode. The calcium produced at the cathode and diffused into the bulk of the cathode thus chemically reducing the TiO2, will form CaO which must become soluble in the electrolyte (CaCl2) and diffuse out of the cathode before additional calcium can diffuse into the inner portion of the cathode for the chemical reduction.
It is also known from x-ray diffraction of the cathode that calcium titanite (CaTiO3) forms as the TiO2 is reduced. A possible reaction is O2−+Ca2++TiO2═CaTiO3 which remains as a contaminate in the cathodically reduced TiO2 to Ti.
U.S. Pat. No. 6,540,902 B1 to Redey teaches that a dissolved oxide in the electrolyte is required to cathodically reduce a metal oxide such as UO2. The example is Li2O in LiCl and the oxygen-ion species is dissolved in the electrolyte for transport to the anode which is shrouded with a MgO tube to prevent back diffusion of oxygen. It is reported the cathodic reduction of the oxide (examples UO2 and Nb2O3) may not take place if the cathode is maintained at a less negative potential than that which lithium deposition will occur. The electrolyte (LiCl) should contain mobile oxide ions which may compress titanium oxide whose concentration of the dissolved oxide species are controlled during the process by controlled additions of soluble oxides. Which titanium oxide is not defined, however, as there are a plethora of different titanium oxides. It is generally known titanium oxides are not soluble in molten salts which accounts for the fact titanium is not electrowon from an oxide feed analogous to aluminum being electrowon from the solubility of Al2O3 in cryolite/sodium fluoride. While the Redey patent teaches cathodic reduction of UO2 and Nb2O3 in a LiCl/Li2O electrolyte, no residual oxygen concentrations are given in the cathode but it was estimated the reduction was 90% complete and no teaching is suggested TiO2 would be reduced to very low oxygen levels.
International publication WO 03/046258 A2 to Cardarelli, assigned to Quebec Iron and Titanium Inc. (QIT) provides a review of electrolysis processes to produce titanium including Fray et al. This patent publication teaches a process analogous to Fray et al. except the process is carried out at a temperature above the melting point of titanium which is approximately 1670° C. A liquid slag containing titantia is used as a cathode on a cell bottom with an electrolyte such as CaF2 floating on top and in contact with anodes such as graphite. Under electrolysis, the impure metals such as iron are deposited at the molten electrolyte titania slag interface and sink to the bottom of the slag since the iron is heavier. After the iron and/or other impurities are removed, titanium is reportedly deposited at the molten slag electrolyte interface and also sinks through the slag settling to the bottom of the cell for subsequent tapping. Oxygen ions diffuse through the electrolyte to an upper anode of graphite. It is suggested the overall reaction is TiO2 (liquid)+C (solid)=Ti (liquid)↓+CO2 (gas)↑.
No specific oxygen residual in the harvested titanium is provided.
Thus, current TiO2 cathode electrolytic processes are no more commercially viable than earlier electrolytic processes.
It is known that metals can be won from their oxide ores by heating with a reductant which typically is carbon. Carbothermic reduction has been established as the most economical process to produce a metal in its pure metallic form. However, carbothermic reduction is not always possible to win a metal from its ore due to not sufficiently reducing impurities within the ore and/or not fully reducing the oxide which may lead to forming the carbide versus complete reduction of the metal oxide. Thus, oxides such as alumina (Al2O3) have not produced pure aluminum by carbothermic reduction. Similarly TiO2 heretofore has not been carbothermically reduced to produce pure titanium. However, in our co-pending parent application, U.S. Ser. No. 10/828,641, filed Apr. 21, 2004, we describe how TiO2 could be carbothermically reduced to TiO. Further investigations have shown it is possible to carbothermically remove more oxygen from the TiO to produce a suboxide of titanium, i.e., having a ratio of oxygen to titanium less than one. The more oxygen removed by the highly efficient and low cost carbothermic reduction, the less required to be removed by electrons in electrolytic reduction which frequently is quite inefficient. Thus the carbothermic reduction of TiO2 as the first process step of producing titanium from TiO2 is enabling.
Titanium is the fourth most abundant metal in the Earths' crust in several mineral forms. The most common utilized minerals are rutile (TiO2) and ilmenite (FeTiO3). Calcium titanates are also an abundant source which contains the element titanium. Utilized as mined or purified through various leaching and/or thermal processing's TiO2 is the most utilized compound which has applications as pigment and for carbo chlorination to produce TiCl4 which is reduced with metals such as magnesium (Kroll Process) or sodium (Hunter Process) that produces titanium metal or the chloride is oxidized to produce a highly purified pigment.
Titanium exists in multivalent species of Ti+4, Ti+3, and Ti+2 in various anionic compositions such as the oxide or chloride. Except for the oxide those compounds are typically unstable in the ambient atmosphere. In general there has been limited application of these subvalent compounds which has not generated processing to produce the subvalent oxides or others compounds.
The high cost of titanium metal has limited its usage to critical aerospace where weight reduction over rides cost sensitivity. Because of the high cost of producing titanium by the Kroll or Hunter processes the cost volume ratio of titanium has tended to be inelastic. The holy grail of titanium is to reduce the cost of the primary metal as well as down stream processing cost. Initiatives are known to be underway to improve efficiency and reduce cost of the basic Kroll and Hunter processes as well as alternative processing involving electrolytic processing. It is known as stated above the FFC Cambridge process which cathodically reduces TiO2 in a calcium chloride process is under development to reduce the cost of primary titanium. It is also known that calcium titanate also forms in this process which limits the process commercial viability. It is also known if cathodic reduction were conducted with a titanium suboxide such as TiO the calcium titanate problem would be eliminated as there is insufficient oxygen to straight forwardly form calcium titanate. It is also generally known that thermal reduction of metal oxides is more economical than using electrons produced by electrolysis which is why iron and many other metals are won by thermal reduction processes.
Since the initiation of the Kroll process to produce titanium in the mid twentieth century, it has been predicted titanium would be produced by an electrolytic process and that process would be similar to the Hall process to produce aluminum. The latter process consist of alumina (Al2O3) exhibiting solubility in fused cryolite (Na3AlF6) which is electrolyzed with a carbon anode that produces CO2 with some CO and the metal aluminum. However, no equivalent process has been developed for solubalizing TiO2. It is possible; however, that the suboxides of titanium can exhibit solubility in some fused salts that may include the alkali, alkaline earth and rare earth halides. However, no reliable low cost process has been available to produce the titanium suboxides that could be used as a feed to electrolytically produce titanium. The titanium suboxide could be utilized cathodically and electrolytically reduced to titanium metal without the calcium titanate problem when using TiO2, and the titanium suboxide could be dissolved in fused salts with electrolysis with a carbon or inert anode to produce titanium. Either processing extreme can produce titanium more economically then the Kroll or Hunter processes. The enabling requirement to produce titanium by these electrolytic processes is a low cost source of titanium suboxides.